Fluid evolution in the Strange Lake granitic pluton, Canada: Implications for HFSE mobilisation

Abstract

Strange Lake is a mid-Proterozoic peralkaline granite pluton (Québec-Labrador, Canada) that underwent extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE). The HFSE mineralisation is confined to highly altered pegmatites and the most altered parts of the granites, implying a genetic association between hydrothermal fluids and HFSE enrichment. This study uses analyses of fluid inclusions to investigate the hydrothermal evolution of the Strange Lake pluton and the role of hydrothermal processes in concentrating the HFSE to potentially exploitable levels.Five groups of inclusions were distinguished. From earliest to latest, these groups are: primary aqueous inclusions (~25wt% NaCl eq.) associated with melt inclusions (Group 1); primary aqueous inclusions (13–23wt% NaCl eq.) associated with CH4 inclusions (Groups 2a and b); primary aqueous inclusions (~9wt% NaCl eq.) associated with CO2 inclusions (Group 3); primary aqueous inclusions (~9wt% NaCl eq.), which contain no carbonic component (Group 4); and finally, secondary aqueous inclusions (19wt% NaCl eq.), including inclusions that outline mineral pseudomorphs, also with no detectable carbonic component (Group 5). Most of the inclusions (except those in Group 5) have conspicuous ‘implosion’ haloes, evident as numerous tiny voids. This indicates that the inclusions re-equilibrated, most likely during isobaric cooling.Fluid evolution commenced with the exsolution of a saline aqueous liquid (~25wt% NaCl eq.) and an immiscible CH4+H2 gas from the pegmatitic melt at temperatures of ~450–500°C and a pressure of ~1100bars. During isobaric cooling, the gas component of the fluid was gradually oxidised, evolving from being CH4-dominant to a CH4 fluid with a significant proportion of higher order hydrocarbons (due to oxidative coupling of methane induced by the consumption of O2 through the alteration of arfvedsonite to aegirine; ~325–360°C), and finally to a CO2-dominated fluid at ~300°C. The apparent salinity of the aqueous fluid decreased from ~25 to ~4.5wt% NaCl eq. due to fluid-rock interaction. The latter also caused precipitation of nahcolite (as a result of the reaction of newly formed CO2 with sodium from decomposing minerals), formation of pseudomorphs after primary Na-zirconosilicates and Na-titanosilicates and replacement of primary REE-silicates by bastnäsite-(Ce). Owing to this interaction, the carbonic component of the fluid was consumed which, together with the consumption of H2O to form Al-, K- and Fe-phyllosilicates, contributed to an increase in the fluid salinity (up to ~19wt% NaCl eq.).Light rare earth elements (LREE) were remobilised over 10s to 100s of metres by the high temperature high salinity fluid (preserved as Group 1 and 2a inclusions), whereas heavy rare earth elements (HREE) were remobilised on a much smaller scale by a late, low temperature high salinity fluid (trapped as inclusion Group 5). Fluid preserved as inclusion Group 3 may have been responsible for remobilisation of Zr and Ti.